This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Time-resolved Studies of Molecular Excited States and Chemical Reactions Beamtime is requested for photocrystallographic time-resolved experiments using single- and multi-pulse Laue diffraction. As part of the project experimental and data-reduction methods are being revised to maximize the accuracy of the Laue intensities and the optical resolution. We are using the 'seed-skewness'method of spot integration (Bolotovsky &Coppens J. Appl. Cryst. 30 244-253 1997)which is profile-independent and well suited for the profile changes observed at different time point in the 100ps-1 ms delay range in time-resolved Laue experiment. We are analyzing instabilities in the single-pulse intensities in collaboration with Tim Graber who has identified several sources of the fluctuations in very recent test experiments. Knowledge of the geometry changes of molecules on excitation and their relation to lifetimes and adsorption of chromophores on substrates is of crucial importance for the design of molecular devices used in light capture. In photovoltaic cells sensitizer-dye molecules are adsorbed on a semiconductor surface which is typically composed of the anatase phase of titanium dioxide. The proposed work involves crystalline phases of titanium dioxide nanoclusters which reproduce the surface characteristics of the anatase phase. The periodic arrangement of the nanoclusters in these materials allows detailed X-ray diffraction determination of the geometry of the adsorbed molecules in their ground state and by use of ultrafast time-resolved pump-probe diffraction methods at picoseconds-resolution determination of the geometry changes of both the adsorbent and the substrate on excitation by light. The structural results are to be correlated with spectroscopic measurements and theoretical calculations to obtain atomic-resolution understanding of the processes that take place on molecule-coated semiconductor surfaces as a result of light exposure. A second goal of the project is the study of initial stages of chemical reactions of molecules in neat crystals as well as of of molecules embedded in framework supramolecular solids. Reactions of interest include isomerizations such as the trans-cis isomerization triggering the reaction of photoactive yellow protein dimerization and ring closure reactions. Whereas the relatively slow progress of such reactions can be studied with conventional equipment and laser excitation the initial stages and in particular the nature of short-lived transition states and other intermediates requires pump-probe techniques with pulsed X-ray and light sources. The work is funded by DOE (first part) and by NSF (second part).